Hybrid electrospinner for core-shell fiber fabrication

ABSTRACT

Electrospinning (ES) provides the technical community with a readily available method to produce polymer fibers ranging from nanoscale to microscale. Here, we present a novel “hybrid electrospirming apparatus,” whereby, modifications to a melt electrospinner have allowed fabrication of core-sheath fibers with polymer sheaths and solution-based cores. These modifications include a split polymer melt heating block, coaxial block spinneret equipped with heaters and multiple feed ports for core and sheath material, and a wiring system for heat which requires multiple switches for safety and on-demand heat activation. Successful demonstration of coaxial fiber fabrication is demonstrated using polycaprolactone-polyethylene oxide blend shell and fluorescent gelatin core materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation application claims the benefit of U.S. NonprovisionalApplication No. 16/402,881 filed on May 3, 2019, which claims thebenefit of U.S. Provisional Application No. 62/666,475 filed on May 3,2018, the disclosures of which are hereby incorporated by reference intheir entirety to provide continuity of disclosure.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Research was sponsored by the Army Research Laboratory and wasaccomplished under Cooperative Agreement Number W911NF-15-2-0020. Theviews and conclusions contained in this document are those of theauthors and should not be interpreted as representing the officialpolicies, either expressed or implied, of the Army Research Laboratoryor the U.S. Government.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTINGCOMPACT DISC APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

Electrospinning (ES) provides the technical community with a readilyavailable method to produce polymer fibers ranging from nanoscale tomicroscale. ES produces fibers with small cross-sections and highsurface area, making them ideal for a multitude of applications.Structures produced using ES methods exhibit a high surface-to-volumeratio, tunable porosity, and controllable composition. ES is of interestto the technical community in areas involving novel ES methods andmaterials including enhanced filtration [D. Aussawasathien, et al.,Journal of Membrane Science, 2008, R. Gopal et al., Journal of MembraneScience, 2007. K. M. Yun, et al., Chemical Engineering Science, 2007. X.H. Qin, et al., Journal of Applied Polymer Science, 2006] augmentedbiomedical tissue regeneration [D. Liang, et al., Advanced Drug Reviews,2007, Kim et al., Biomaterials, 2003], and advanced fabrication ofliquid crystal polarizers [Y. YF, et al., Advanced Materials, 2007].Although ES is the common term today, it was initially described byFormhals in a series of patents as an experimental setup for theproduction of polymer filaments using electrostatic force. The firstpatent filed by Formhals in 1934 on ES was issued for the production oftextile yarns, with a process consisting of a movable thread collectingdevice that gathered threads in a stretched condition. He was grantedrelated patents in 1938, 1939, and 1940[K. J. Pawlowski, et al.,Materials Research Society Symposia, 2004]. ES was first observed in1897 by Rayleigh, with related electrospraying studied in detail in 1914and a patent issued to Antonin Formhals in 1934 [J. Zeleny, PhysicalReviews, 1914, A. Formhals, Patent US1975504A, 1934]. In 1969, thepublished work of Taylor set the foundation for ES [G. Taylor,Proceedings “Electrically driven jets,” Proceedings of the Royal Societyof London A: Mathematical, Physical, and Engineering Sciences, 1969].

ES involves the delivery of a liquid polymer to a spinneret (sometimesreferred to as a capillary or needle) [I. S. Chronakis, Journal ofMaterials Processing Technology, 2005, Z. M. Huang, et al. Journal ofComposites Science, 2003, J. Doshi et al., Journal of Electrostatics,1995] that is held at a high voltage relative to a collection plate [J.L. Skinner et al., Proceedings of SPIE—The International Society forOptical Engineering, 2015]. Polymer is pumped to the tip of thespinneret, and electric charge is initiated in the collection plate. Theinitiated voltage creates an electrostatic force that pulls polymer fromthe spinneret to an electrode deposition surface. An initial shortregion (microns to millimeters) where the fiber is essentially straightis called the stable region. At the point where lateral perturbationscause transverse fiber velocities, the instability region starts. Theinstability region consists of polymer fiber moving in a whipping motionfrom the stable region toward the collection plate, while solventevaporates off the polymer jet. Polymer fibers are then deposited ontothe collection surface. Fiber size depends largely on solution flowrate, supplied electric current, and fluid surface tension [S. V.Fridrikh, et al., Physical Reviews Letters, V. Beachley et al.,Materials Science Engineering C, 2009, A. Koski, et al., MaterialsLetters, 2004]. Given the time scales associated with fiber depositionby ES, charges on the metallic collection plate move instantaneously.Motion of charge in the polymer (much slower than motion of charge inmetals) is dictated by ionic mobility in the polymer [D. H. Reneker, etal., Journal of Applied Physics]. Any effort to control the electricfield within ES must take into account the high-frequency cutoffenforced by polymer limitations. The low-frequency cut-off for dynamicfield control relates to the spatial fiber deposition rate and timeconstants associated with the instability region.

In solution ES, polymers which are pre-dissolved in solvent are used.Although solution ES is more common, melt ES can also be performed, inwhich, solid polymers are used. Melt-ES does not require solventevaporation, instead creating a liquid polymer from a solid, in whichphase transition of the polymer is associated with increased temperature[P.D. Dalton, et al., Biomacromolecules, 2006, J.S. Kim et al., PolymerJournal, 2000, L. Larrondo et al., Journal of Polymer Science andPolymer Physics, 1981, S. Lee et al., Journal of Applied PolymerScience, 2006, J. Lyons, et al., Polymer, 2004]. Lack of solvent in meltES is beneficial for two reasons. First, the lack of harsh solventrequires less precaution during polymer preparation, and second, theinstability region caused by solvent evaporation is a non-issue. Melt-ESwas first patented by Norton in 1936 [C. L. Norton, US Patent2048651,1936] but it was not until 1981 that a three-paper seriesdescribing electrostatics and polymer melts was published by Larrondoand Manley [L. Larrondo, et al., Journal of Polymer Science Part B,1981].

BRIEF SUMMARY OF THE INVENTION

An apparatus designed for hybrid ES, whereby, materials which consist ofcore-sheath fibers can be fabricated is disclosed herein. The hybridelectrospinner designed allows a polymer melt to encase a solution-based(solution or solution-based) core, forming coaxial or core-sheathstructured fibers. ES in the dual feed system designed will forcepolymer shell and core solution into a high voltage electric fieldgenerated between the spinneret and the collection plate. The electricfield described is initiated by an externally applied voltage. Theelectric field used during ES creates a force on polymer shell and coresolution, which results in deformation of the polymer/solution stream tolower surface area. Polymer shell and core solution are then pulled bythe electrostatic force from spinneret to collection plate, formingcore-shell fibers with a solid shell and liquid core.

For core-shell or coaxial structures to be fabricated on melt ES, anovel ES spinneret had to be designed. Because melt ES involves drypolymers, the spinneret had to be equipped not only with concentricspinneret configuration, but also heaters to melt the dry polymer usedfor the shell, prior to ES. The novel ES spinneret designed is comprisedof an outer and inner annulus contained within a block spinneret. Theblock spinneret also contains two channels for cartridge heaters and achannel that incorporates a feed control mechanism for the outer annulusregion.

The hybrid electrospinner heating design will include cartridge heatersand a Proportional-Integral-Derivative (PID) controller. In the heatingdesign, two safety switches were added to require the main power inputcontroller and also a secondary control switch on the heaters. Thesecondary control switch enables powering of the PM controller, but notactivating the heaters until desired.

The fiber sheath created during ES with the hybrid electrospinner isdelivered to the outer annular region of the coaxial block spinneretfrom a split block used to melt the polymer prior to ES. The split blockdesign allows polymer to be molded to the correct shape for the hybridelectrospinner, and also for easy cleaning. The split block melt chambercontains a threaded fitting to attach to the sheathing feed of thespinneret, and an isolated heater cartridge holder to maintain the heatinput but separate fed polymer from the cartridge heater.

The core (solution) feed for the hybrid electrospinner was designed toprovide control over flow rate and provide consistent flow through thecoaxial block spinneret within the applied electric field withoutdamaging any electronics. This was accomplished by feeding the coresolution through a syringe regulated by a commonly used syringe pump.

The hybrid electrospinner was designed and fabricated to electrospinmonoaxial polymer fibers, or coaxial (core-sheath/shell) fibers with asolution-based core and polymer shell.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic of the design used for the hybrid ES apparatus.The set-up shown is designed to enable production of materials whichcontain fibers composed of a polymer melt sheath and a solution- basedcore, although monoaxial polymer fibers can be electrospun as well.

FIG. 2 is a schematic plan view of a coaxial (core-sheath) spinneretdesign incorporating two heaters to melt polymer prior to ES.

FIG. 3 is a perspective view of a coaxial (core-sheath) spinneret designincorporating two heaters to melt polymer prior to ES.

FIG. 4 : is a photograph of a coaxial block spinneret prototype. Imageshows two ports for heater cartridges as well as two feed ports. Sideport allows polymer feed for fiber sheath, while top port is used forsolution, core feed.

FIG. 5 is a wiring schematic for a PID heater controller. The controlloop shown has two main power controls, one for overall power, and asecond for heater power. The separated control design allows the PIDcontroller to be activated and set without activating the heaters untildesired.

FIG. 6 is a photograph of a PID controller and power switch controlpanel.

FIG. 7 is a perspective transparent view of the polymer melting chamberdesign with split block.

FIG. 8 is a perspective view of the split block design of the meltingchamber, which allows for polymer melt to be molded for delivery intothe outer annulus of the coaxial block spinneret for sheath materialdelivery, as well as for easy cleaning.

FIG. 9 is a photograph of the split block melt chamber attached to theside of the coaxial block spinneret. Here, heater cartridges have beeninserted into the melt chamber as well as the spinneret to provide heatat multiple points along the polymer path prior to extrusion from thespinneret and ES.

FIG. 10 is a photograph of a syringe pump used to deliver solution coreto the coaxial block spinneret. The syringe pump allows for control overthe core solution delivery to the spinneret using a stepper motor forcalibrated flow rate set by the user.

FIG. 11 is a far field micrograph of monoaxial electrospun fibersproduced from polypropylene melt in the hybrid electrospinner. DuringES, polypropylene melt was subjected to an electric field strength of367 kV/m. The resulting fibers had an average diameter of 20 μm andpresented a smooth, consistent surface.

FIG. 12 is another far field micrograph of monoaxial electrospun fibersproduced from polypropylene melt in the hybrid electrospinner. DuringES, polypropylene melt was subjected to an electric field strength of367 kV/m. The resulting fibers had an average diameter of 20 μm andpresented a smooth, consistent surface.

FIG. 13 is a photograph taken during ES. Photo shows polymer/solutionjet deforming into a cone shape. This is typical during ES, wheredeformation incurs as a mechanism to reduce surface area during electricfield exposure. Solution core can be seen pulling into the polymerstream, while polymer sheath material maintains the overall structure ofthe stream, which will result in coaxial fiber formation.

FIG. 14 is an epifluorescent micrograph showing coaxial electrospunfiber containing a polycaprolactone/polyethylene oxide blend polymershell and fluorescent gelatin core. The fiber produced was fabricated inan electric field strength of 400 kV/m, and a diameter of approximately20 μm with a core diameter of 9 to 15 μm.

DETAILED DESCRIPTION OF THE INVENTION

A schematic of a hybrid electrospinner is shown in FIG. 1 . Said hybridelectrospinner is comprised of: a heating block 30, which is furthercomprised of a polymer melt chamber 1, and at least one heater cartridge2; a hybrid spinneret 3; a syringe pump 4; a thermocouple 5; atemperature controller 6; a high voltage power supply 7; and acollection plate 9. Whereby said heating block 30 is further comprisedof a communication 10 between said polymer melt chamber 1 and saidhybrid spinneret 3 for delivery of polymer melt 11. Said at least oneheater cartridge 2 is designed to be inserted and removable from saidheating block 30. Said syringe pump 4 delivers core solution 13 to thehybrid spinneret 3. The design shown allows a polymer melt 11 to encasea core 14, where said core can be solution-based or solution, formingcoaxial fibers or core-sheath structured fibers 15. ES in this dual feedsystem will force polymer shell 17 and core solution 13 into a highvoltage electric field 18 generated between the hybrid spinneret 3 andthe collection plate 9. The electric field 18 described is initiated byan externally applied high voltage power supply 7. The electric field 18used during ES creates a force on polymer shell 17 and core solution 13,which results in deformation of the polymer/solution stream to lowersurface area. Polymer shell and core solution are then pulled by theelectrostatic force from the hybrid spinneret 3 to the collection plate9, forming core-sheath structured fibers 15 with a solid shell andliquid core as shown in FIGS. 13 and 14 .

For coaxial or core-sheath structured fibers 15 to be fabricated by meltES, a novel ES hybrid spinneret 3 had to be designed. Solution ES ofcoaxial structures involves a simple change of the spinneret to containone instead of two concentric spinnerets, which are each fed a separatepolymer or solution. Because melt ES involves dry polymers, the hybridspinneret 3 had to be equipped not only with concentric spinneretconfiguration, but also heater cartridges 2 to melt the dry polymer usedfor the shell, prior to ES. The novel ES hybrid spinneret 3 designedcontains an outer annular wall 19 and inner annular wall 20 containedwithin the hybrid spinneret. Melted polymer enters the hybrid spinneret3 through a communication 10 comprising a polymer entry port 21,allowing polymer flow and fiber sheath formation within the spacecreated between the outer annular wall 19 and inner annular wall 20.Core solution-based material entry 22 occurs via a core solution feed31, which allows core solution to flow into the space enclosed by theinner annular wall 20. The outer annular wall 19 and inner annular wall20 terminate at an extrusion port 23 where the polymer andsolution-based material are subjected to the electric field 18 for ES ofcore-sheath structured fibers 15. The hybrid spinneret 3 also containstwo channels 24 for cartridge heaters 2 to be removably inserted, and achannel that incorporates a feed control mechanism for the outer annularregion.

A schematic model of the coaxial melt ES hybrid spinneret 3 is shown inFIGS. 2 and 3 , while a photograph of the hybrid spinneret 3 is shown inFIG. 4 .

The hybrid electrospinner heating design includes heater cartridges 2and a temperature controller 6 comprising aProportional-Integral-Derivative (PID) controller 25. The wiring diagramfor the heating mechanism designed is shown in FIG. 5 . In FIG. 5 , acompleted heater control loop is shown, excluding the thermocouple 5that the PID controller 25 collects data from. FIG. 5 schematic containstwo 144 Ohm heater cartridges 2, a PID controller 25, a heater powerswitch 26, a main power switch 27 and a high voltage power supply 7comprising a 120-volt power source. The heater cartridges 2 arecomprised of HDC0031 heaters that are matched up to an Omega PIDcontroller 25 for temperature control. Also seen in the wiring schematicare the two switches added as a safety precaution, requiring not onlythe main power input controller, but also a secondary control switch onthe heaters. The secondary control switch enables powering of the PIDcontroller, but not activating the heaters until desired. Controllerswitches 26 and 27 on the panel are shown in FIG. 6 .

The feed mechanism for the fiber sheath creating during ES is shown inFIGS. 7 and 8 . The design shown is comprised of a heating block 30 witha polymer melt chamber 1, which provides a feed 28 to the outer annulus19 region of the hybrid spinneret 3. Said feed 28 can be threaded toaccommodate a threaded fitting 33, for attachment to the polymer entryport 21 of the hybrid spinneret 3. The heating block 30 design comprisesa split block shown in FIG. 8 so that melt polymer mold can be preparedin the melt chamber 1 mold, and also for ease of cleaning. The heatingblock 30 is also comprised of an isolated melt chamber heater cartridgechannel 29 to accept a melt chamber heater cartridge, which maintainsheat input, but is separate from the fed polymer. In FIG. 9 , aphotograph of the heating block 30 is shown attached to the side of thehybrid spinneret 3. Active control or gravity feed are both possiblewith the feed mechanism designed.

The core solution feed 31 for the hybrid electrospinner was designed toprovide control over flow rate and provide consistent flow through thehybrid spinneret 3 within the applied electric field 18 without damagingany electronics. This was accomplished by feeding the core solutionthrough a syringe 32 regulated by a commonly used syringe pump 4 shownin FIG. 10 .

The hybrid electrospinner was designed and fabricated to electrospinmonoaxial polymer fibers, or coaxial (core-sheath/shell) fibers with asolution core and polymer shell. In FIGS. 11 and 12 , an example ofmonoaxial fibers electrospun with the hybrid electrospinner are shown.In FIGS. 13 and 14 , an example of coaxial fibers electrospun with thehybrid electrospinner are shown.

It is understood that the foregoing examples are merely illustrative ofthe present invention. Certain modifications of the articles and/ormethods may be made and still achieve the objectives of the invention.Such modifications are contemplated as within the scope of the claimedinvention.

1. A Hybrid ES apparatus comprising: a. a heating block, comprised of apolymer melt chamber further comprised of a feed at a bottom end of saidpolymer melt chamber, and at least one heating block heater cartridgechannel isolated from said polymer melt chamber to accept at least oneremovable heating block heater cartridge; b. a hybrid spinneretcomprised of a concentric spinneret further comprising an outer annularwall and an inner annular wall, where a space is created between saidouter annular wall and inner annular wall to accept a melted polymerfrom said polymer melt chamber feed through a polymer entry port of saidhybrid spinneret where said melted polymer enters said space betweensaid outer annular wall and inner annular wall, and a space is createdinside said inner annular wall isolated from said space between saidouter annular wall and inner annular wall, which accepts a core solutionfrom a core solution feed located at a first end of said hybridspinneret; where said outer annular wall and inner annular wallterminate at a polymer extrusion port located at a second end of saidhybrid spinneret; and whereby said hybrid spinneret is further comprisedof at least one removable spinneret heater cartridge channel isolatedfrom said outer annular wall, which accepts at least one removablespinneret heater cartridge.
 2. The Hybrid ES apparatus of claim 1 wheresaid feed and said polymer entry port are threaded to accommodate athreaded fitting for attachment of said heating block to said hybridspinneret.
 3. The Hybrid ES apparatus of claim 1 where a syringe and asyringe pump delivers core solution to the space created by said innerannular wall.
 4. The Hybrid ES apparatus of claim 1 where said heatingblock comprises a split block to facilitate cleaning.
 5. The Hybrid ESapparatus of claim 1 further comprising a temperature controllercomprised of a Proportional-Integral-Derivative (PID) controller, whichcontrols the temperature of said heating block and said hybridspinneret.
 6. The Hybrid ES apparatus of claim 5 further comprising amain power input controller, and a secondary control switch for saidheater cartridges to enable powering of said PID controller withoutactivating said heater cartridges until desired.